This invention relates to Cofxe2x80x94Craxe2x80x94Nigxe2x80x94Ptbxe2x80x94Bcxe2x80x94(Si,Zr,Fe,W,Mo,Sm)dxe2x80x94Tae magnetic target materials and more particularly relates to methods for production of magnetic target materials with chemically homogeneous microstructures which promote sputter deposited films with higher and more uniform magnetic coercivities than films deposited using similar target products known to the art.
a=0 to 60 atomic %.
b=0 to 20 atomic %.
c=0 to 15 atomic %.
d=combination of one or more of these elements not to exceed 30 atomic %.
e=0.5 to 6 atomic %.
g=0 to 40 atomic %.
f=remainder.
Data storage disks used in computer hard drives are manufactured by magnetron or RF sputter deposition processes. The disk itself comprises several different layers of material. Typically, thin-film disk technology uses a Al blank as the base substrate material with a hard amorphous Nixe2x80x94P layer with thickness of about 10 microns electrolessly plated onto it. Johnson et al., IBM J. Res. Develop., Vol. 40, Nos. 5, September 1996, p. 511-536. The Nixe2x80x94P layer is scribed with fine texturizing grooves. An underlayer of Cr with thickness between 20 to 100 nm is sputter deposited onto the Nixe2x80x94P layer to ensure nucleation of the magnetic film with the easy axis of magnetization in-plane for longitudinal recording. A Co-based magnetic film, with composition in the ranges"" described above, is sputter deposited onto the underlayer with thickness"" of about 30 nm for magnetoresistive head applications. Finally, a 10-20 nm protective layer of hydrogenated carbon is reactively sputtered on top of the magnetic layer. Alternate substrate (i.e. glass) and underlayer materials (i.e. Crxe2x80x94V, Nixe2x80x94Al, Crxe2x80x94Ti) are commonly utilized in the data storage industry.
Magnetron sputtering to form the magnetic film on the data storage device (disk), involves the arrangement of permanent or electromagnets behind the magnetic target material (cathode). The applied magnetic field transmits through the target and focuses a discharge plasma onto the front of the target. The front of the target surface is atomized with subsequent deposition of the magnetic target atoms on top of the underlayer of the evolving disk. Typically, sputtering of the various non-magnetic and magnetic layers, comprising the architecture of the disk, is conducted on both sides of the disk.
A potential problem which results during the sputtering of Co-based alloys containing Ta additions is the effect of the homogeneity of the multi-phase target microstructure on the resultant magnetic properties of the deposited film. The maximum solid solubility of Ta in Co is 4 atomic % at 1280 Celsius (Massalski et al. xe2x80x9cBinary Phase Diagramsxe2x80x9d, ASM International, Vol. 2, 1990, p. 1245) as depicted in FIG. 1. As other alloying additions are added to Co, the maximum solid solubility of Ta in the matrix is further decreased. Therefore, the driving force for the formation of eutectic Ta-rich particulates in the microstructure of Co-based magnetic alloys is very large. The eutectic Ta-rich particulates have been identified as possessing a Co2Ta stoichiometry, (Schlott et al., IEEE Transactions on Magnetics, Vol. 31, No. 6, November 1995, p. 2818-2820; Massalski et al. supra). Even in the case of alloys possessing Ta contents less than the maximum solid solubility limit, the propensity for Ta-rich second-phase formation is very large since these alloys are typically not thermomechanically processed at the eutectic temperature.
Furthermore, even if the Co-based alloys containing Ta are processed within proximity of the eutectic temperature, Ta-rich second-phase formation will occur if sufficient cooling rates are not employed since the solid solubility limit diminishes rapidly with decreasing alloy temperature.
It is fairly typical that a pair of magnetic alloy targets can be used to fabricate in excess of 10,000 individual data-storage disks. Since the magnetic target alloy is continually losing surface atomic layers during the sputtering process, through thickness and in-plane target microstructural homogeneity is essential to ensure film property homogeneity on the many thousands of disks fabricated from each target and the many thousand more fabricated from the numerous targets constituting a production lot or originating from several individual production lots. A production lot represents all the targets that are exposed to exactly the same thermomechanical history (i.e. originating from one melted ingot or one hot-isostatic-press container).
Ta-rich second-phase segregation in the matrix of Co-based magnetic target alloys has been shown to impact deposited film magnetic properties such as Coercivity. When tens of thousands of data storage devices are being made from several targets, it is necessary that the Coercivity response be consistent on all the disks, i.e., quality control, and not be a function of the specific target utilized. Therefore, there is a substantial need in the art for Ta containing Co-based magnetic targets which exhibit consistent performance, both within a target and from target to target.
Standard production practices for magnetic target alloys involve the following thermomechanical steps. See Schlott et al., supra; U.S. Pat. No. 5,468,305; U.S. Pat. No. 5,334,267; and U.S. Pat. No. 5,282,946. Ingots are fabricated by either casting or Hot-Isostatic-Pressing (HIP ping) of elemental powders. Hot-rolling is then conducted primarily to heal any residual porosity in the ingots and form plates from which the magnetic targets can be obtained. The ingot and plate sizes are pre-determined to enable extraction of the particular target geometry required (i.e. rectangular targets and circular targets possessing a variety of different dimensions). Single-step hot-rolling practices at temperatures between 700F to 2200F are typically employed. After hot-rolling, heat-treatment and cold-texture deformation processes are utilized to reduce the bulk magnetic permeability of the target product. A reduction in bulk magnetic permeability is utilized to improve the efficiency of the sputtering process by facilitating optimum passage of magnetic flux through the bulk of the target. Shunting of magnetic flux within the bulk of the target adversely affects the stability of the sputtering process, material yield of the target and thickness uniformity of the deposited film. Alternate fabrication practices involving water-cooled as-cast target fabrication with no further down-stream thermomechanical processing are also known in the prior art. A review of the prior art reveals that controlling hot-rolling temperatures, or utilization of individual homogenization practices, have not been employed to minimize the formation of Ta-rich second-phase particulates in the target microstructure.
The present invention will focus on the addition of specific homogenization treatments to the fabrication of magnetic target materials in order to minimize and homogenize the presence of Ta-rich second-phase particulates in the microstructure.
It is accordingly one object of the present invention to provide a method for the production of Ta containing Co-based magnetic targets which promote consistent deposited film magnetic property performance.
A further object of the invention is to provide a process for the production of magnetic targets wherein the potential for formation of a course Ta-rich second phase in the microstructure is reduced.
An even further object of the invention is to provide magnetic target alloys wherein the presence of a Ta-rich second phase in the microstructure has been substantially eliminated.
Other objects and advantages of the present invention will become apparent as the description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention provides a process for the production of magnetic target alloy materials which possess chemically and mechanically uniform microstructures both within a target and from target to target, wherein a homogenization and hot-rolling practice is utilized to dissolve the Ta-rich second-phase back into solution. The process involves soaking the ingots from which the targets are produced at temperatures ranging from 1600xc2x0 F. to 2600xc2x0 F. for periods of 10 minutes to 24 hours prior to hot-rolling, optionally using multiple steps, then hot rolling at similar temperatures utilizing at least a 3% reduction per pass, and finally, optionally soaking the rolled plates from which the targets are produced at temperatures ranging from 2000xc2x0 F. to 2600xc2x0 F. for periods of 10 minutes to 24 hours. Note, reduction per pass is defined as: (1xe2x88x92To/Ti)xc3x97100%, where Ti is the thickness of the ingot/plate input into the rolling mill and To is the thickness after rolling by one pass. Directly following the thermomechanical processing, the cooling rate of the plate must be equal to or faster than an air cool. A cold water quench is preferable. It has been discovered that this practice will retard the formation of Ta-rich second-phase and result in a Co-based magnetic target alloy wherein the tantalum second-phase is substantially minimized or completely eliminated. Also provided by the present invention are target alloys depicting the beneficial effect of a homogenized magnetic target alloy microstructure on the magnetic Coercivity of the resulting sputter deposited thin film.